Refrigerant vapor compression systems are well known in the art and are commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in refrigerating air supplied to display cases, merchandisers, freezer cabinets, cold rooms or other perishable/frozen product storage area in commercial establishments.
Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable/frozen items by truck, rail, ship or intermodally. Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature. The desired temperature at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved. The refrigerant vapor compression system must not only have sufficient capacity and refrigerant charge to rapidly pull down the temperature of product loaded into the cargo space at ambient temperature, but also operate efficiently at low load with excess refrigerant charge when maintaining a stable product temperature during transport.
In transport refrigeration applications, the refrigerant vapor compression system needs to generate sufficient cooling to chill the cargo space to a preselected set point temperature. For perishable food products, the cargo space would be cooled to a set point temperature of slightly above 0° C. (32° F.) to as high as about 30° C. (86° F.), depending upon the particular perishable food product being transported. However, for frozen food products, the cargo space would be cooled to a set point temperature below 0° C. (32° F.) to as low as −20° C. (−4° F.) or lower, depending upon the particular frozen food product being transported. Due to this wide range of product temperature requirements, the cargo space of a truck, trailer, container or the like is typically a single compartment housing either a perishable product, such as fruits or vegetables, or a frozen product, such as meat or seafood products. Therefore, most refrigerant vapor compression systems are simply controlled in response to a single set point temperature.